The proposed collaborative project is based collectively on investigations of the pathogenesis and epidemiology of Lyme disease, the ecology, vector biology, and population genetics of vector-borne diseases, the molecular biology of Borrelia burgdorferi, and vaccine development. The emphases of the proposed studies are the evolutionary biology of B. burgdorferi and new strategies for Lyme disease prevention. The long term goals are the following: (1) To significantly reduce the risk of Lyme disease among humans and domestic animals by vaccine intervention in the natural maintenance cycle of B. burgdorferi in a highly endemic area. (2) To study the evolution of B. burgdorferi as an infectious agent through controlled intervention studies in a highly endemic area. (3)To provide for a vaccine that is highly effective and that can be expeditiously, safely, and inexpensively administered in the field. The hypotheses for the project are the following: (A) A high prevalence of anti-OspA antibodies among Peromyscus leucopus and/or other mammalian reservoirs in the field will reduce the transmission rate between reservoir hosts and vector ticks, thereby reducing the prevalence of B. burgdorferi infection in ticks. (B) Immunization of reservoirs in the field will not alter the population structure of B. burgdorferi at the vaccine field sites in comparison to the control sites. (C) Transmission-blocking immunity among P. leucopus and/or other mammalian reservoirs can be achieved by oral as well as by parenteral immunization with a single immunogen or combined immunogens. The specific aims are the following: (1) Field studies of the effect of immunization of P. leucopus on the rate of transmission of B. burgdorferi between reservoir host and vector ticks. (2) Assessment of the effects of vaccine intervention in the field on the population structure of B. burgdorferi in ticks and in vertebrate reservoirs. (3) Further development of single-vaccination and orally-delivered field vaccines and evaluation of second, supplementary antigens. The proposed studies may provide insights as molecular, organismal, and population levels about B. burgdorferi, its transmission, and reservoir host immunize responses. The results may also have relevance for the control of other vector-bone zoonotic diseases and perhaps for development of novel methods for vaccine delivery.